virtio_console: avoid config access from irq

commit eeb8a7e8bb123e84daeef84f5a2eab99ad2839a2 upstream.

when multiport is off, virtio console invokes config access from irq
context, config access is blocking on s390.
Fix this up by scheduling work from config irq - similar to what we do
for multiport configs.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop changes to virtcons_freeze()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>